www.desktopdungeons.net/ (http://www.desktopdungeons.net/)
Don't let the crappy graphics disuade you from trying it. Even after watching a few reviews on youtube I didn't feel much like even trying it. I did and it turns out to be pretty fun :).
Haha, I watched TotalBiscuit's WTF is... on this, then I started playing it, then I got addicted, seriously a great game.
Haha same review I watched.. I have unlocked all the basic classes but I have yet to win the special challenges like snakepit, crypt, library...
Whats fun is that u can play 10 minutes or 10 hours its up to you.